Any amplifier or A/V receiver that I have seen has a black and a red terminal for each speaker. From 2 pairs for a simple stereo system up to 7 or more for an A/V receiver.
It is quite a while since I have looked inside an amplifier but, for at least some, the blacks are common and connected to the chassis. Can I safely assume this or are there any common or likely amplifier designs in which the black terminals might not all be at the same potential?
Edit: Chassis might not have been the best word; as people seem to have picked up, the significance is whether they are all necessarily at the same potential. My outlook is still biased by early experience with valve devices: radios, TVs, amplifiers, in which a metal chassis (not an external casing) was usually a common 0V reference.